1. Field of the Invention
The present invention relates to an improved process and catalytic mixture for use in the oxidation or ammoxidation of a hydrocarbon to the corresponding product. In one embodiment, the present invention is directed to an improved process for the ammoxidation of propylene, isobutylene, propane, isobutane or mixtures thereof, to acrylonitrile and/or methacrylonitrile. In another embodiment, the present invention is directed to an improved process for the oxidation of propylene, isobutylene, propane, isobutane or mixtures thereof, to acrylic acid and/or methacrylic acid. In these embodiments, the oxidation or ammoxidation occurs in the presence of a molybdenum-based catalyst and a catalyst modifier. More specifically, the modifier can be added to the catalyst in situ, to modify the performance of the catalyst in the reactor and inhibit molybdenum oxide loss from the base catalyst.
2. Description of the Prior Art
Catalysts containing oxides of bismuth and molybdenum, promoted with suitable elements, have long been used for the conversion of propylene at elevated temperatures in the presence of ammonia and oxygen (usually in the form of air) to manufacture acrylonitrile. U.S. Pat. Nos. 5,093,299, 5,212,137, 5,658,842 and 5,834,394 are directed to bismuth-molybdenum promoted catalysts exhibiting high yields to acrylonitrile. Great Britain Patent 1,436,475, U.S. Pat. Nos. 4,766,232, 4,377,534, 4,040,978, 4,168,246, 5,223,469 and 4,863,891 are each directed to bismuth-molybdenum-iron catalysts that may be promoted with the Group II elements to produce acrylonitrile.
Typically, the yield of acrylonitrile is in the upper 70% per pass conversion range, with the remaining products including primarily hydrogen cyanide (HCN), acetonitrile, acrolein, acrylic acid, and carbon oxides. The co-production of HCN and acetonitrile has led to the development of ancillary businesses for these products. The balance of demand for each of these nitriles can dictate the economics for catalyst yields at a given production site. Thus, one production facility may desire to produce more HCN than another.
One approach that has been taken to increase the yield of hydrogen cyanide is to select the operating conditions. However, changing the operating conditions to increase the yield of hydrogen cyanide has always led to an economically unacceptable decrease in the production yields of acrylonitrile.
Specialty catalysts have been developed that increase the yield of hydrogen cyanide co-product produced during the production of acrylonitrile without economically unacceptable losses in acrylonitrile production. For example, U.S. Pat. No. 5,840,648 describes a catalyst that increases the yield of hydrogen cyanide. However it is costly to replace the catalyst charged to a reactor just to modify the yield of certain co-products. A catalyst modifier that can be added in situ and that can increase the yield of desirable co-products, without economically unacceptable losses in acrylonitrile or methacrylonitrile production, is needed.
The decline of catalyst activity after prolonged exposure to ammoxidation conditions, accompanied by a partial loss of molybdenum oxide from the catalyst is often observed. Not only is there a loss of activity, but there is the possibility that the molybdenum oxide lost from the catalyst will deposit onto low temperature surfaces of the reactor system and become scale that is then difficult to remove. U.S. Pat. No. 6,136,998 describes a catalyst that contains small amounts of tellurium, which appears to inhibit the loss of molybdenum oxide from the catalyst. However, a catalyst modifier that can easily be combined with the molybdenum-based catalyst of choice to inhibit molybdenum loss would be desirable.